Psilocybe allenii is a species of agaric fungus in the family Hymenogastraceae. Described as new to science in 2012, it is named after John W. Allen, who provided the type collection. It is found in the northwestern North America from British Columbia, Canada to Los Angeles, California, most commonly within 10 miles (16 km) of the Pacific coast.
The fruitbodies (mushrooms) grow on rotting wood, especially wood chips used in garden landscaping. The caps of the mushrooms are brown to buff, broadly convex to flattened and have a diameter up to 9 cm (3.5 in), while the white stipes are up to 9 cm (3.5 in) long and 0.7 cm (0.3 in) thick.